1. Field of the Invention
The present invention relates to a preforming method performed prior to simultaneous injection molding for forming a cover member made of synthetic resin. The cover is used for a can-shaped container such as a can for various beverage or canned food container or other similar containers.
2. Background of the Invention
One such can-shaped container of the type described above uses a synthetic resin as a main material, (see Japanese Laid-opened Patent Application Publication No. 52-39489).
The present inventor has been engaged in research on integral metal molding within a metal mold of an injection machine in an injection molding process. Instead of using a conventional adhesive agent procedure various techniques have been proposed, such as those disclosed in Japanese Patent Applications Nos. 58-90749, 58-124756, 59-146946, 59-146943, 59-146944, 59-248082 etc. In these techniques, an adhesive agent is applied to a provisionally injection-molded plastic sheet. A metal foil surrounded on both sides with plastic material layers provides a cover of a can-shaped container formed mainly of a plastic material.
The injection molding method will be described with reference to FIGS. 1-3 These processes have been described in commonly assigned U.S. patent application, Ser. No. 032,125, filed Mar. 30, 1987, now U.S. Pat. No. 4784284 incorporated herein by reference. This reference also discloses but does not claim many of the significant steps of the present invention. As shown in FIG. 1, a base (insert member) 40 having plastic material layer on both its sides or possibly only on one side is moved by a suction plate 41 connected to a movable cylinder 42 of a robot. The insert member 40 is inserted into a guide member (stripper plate) 43. As shown in FIG. 2, the insert member 40 is fixedly secured in the stripper plate 43 which thus prevents positional displacement of the insert member 40. Then, as shown in FIG. 3, the member is subjected to clamp-shaping.
By this clamping operation as shown in FIG. 3, the plate-like (two-dimensional shape) insert member 40 is bent at its peripheral end portion within a metal mold (core type receptacle) 44. Thereafter, molten resin is injected into a mold cavity (mold space) 48 defined between the core mold 44 and a cavity mold 47. The molten resin is injected through a gate 46 of the metal mold (cavity mold, injection mold) having also a resin in flow passage 45, so that can-shaped container cover is obtained.
The above-mentioned simultaneous molding (integral molding) process has the following advantages. (1) The number of production steps is reduced. (2) Peeling during retorting treatment of the food product contained in the can can be avoided because of high bonding strength in the molded product. (3) The resultant product has high resistance to damage if it is dropped (4) Good food sanitation properties is obtainable. Thus, an excellent can-shaped container cover is provided.
Incidentally, in case the flat insert member 40 is clamped and is subjected to bending treatment to obtain the three dimensional configuration as shown in FIG. 3, the insert member 40 may become deformed. The deformation or shearing is primarily in the form of wrinkles. Therefore, irregular and large wrinkles are generated if the insert member 40 is installed into the metal mold while maintaining the flat shape of the insert member 40. This problem may be overcome to some extent by using a relatively large insert member. However, in so doing, another problem may be raised in that the insert member may be sandwiched at the parting line of the confronting molds.
If injection molding is carried out against the insert member having deformation, the resultant molded product may be degraded due to non-uniform deformation.
As will be explained later, FIG. 4 shows a bottom view showing one example of can-shaped container cover (bottom cover) produced by the injection molding described above. In FIG. 4, injected resin may flow along a side wall 23 formed in the insert member 40 so that injected resin 17D adheres to the side wall 23. This resin adhesion is particularly significant at wrinkle generating portions 24 shown in FIG. 4A.
The bottom cover has a flap portion and the side wall. A barrel portion of the can-shaped container is fitted between the flap and the side wall. Further, an upper cover is attached to the can barrel to thereby form the can-shaped container. The upper cover has a construction similar to the lower cover and is produced by the injection molding. The flap portion in the cover is formed of the injected resin. The can barrel has its surface formed with heat-fusible resin layer.
These covers are generally attached to the can barrel by melting the resin through high frequency induction heating. In the heating process, the Al foil of the insert member is heated and the resin layer of the insert member is melted by the heat conduction, thus allowing it to fuse bond with the can barrel.
As described above, if the resin adheres over the side peripheral surface of the bottom cover due to resin outflow, (the same is true with respect to the upper cover), a larger heat amount is required for heating the Al foil and for melting the resin at the side wall. Therefore, the heat generating state may become irregular during high frequency induction heating. As a result, there results non-uniform sealing for attaching the cover to the can barrel. Also, the Al foil may be cut due to undue local heating. Further, leakage of content in the can barrel may occur.